ADU rules in Nampa

A detached ADU is allowed by right in every Nampa residential zone. Ordinance No. 4948 (adopted September 21, 2026) created a new Title 10 chapter 29 whose 10-29-3.C reads 'An accessory dwelling unit may be internal to, attached to, or detached from a stand alone structure', and 10-29-3.A says 'An ADU is permitted within residential zones as defined in 10-3-2'; the 10-3-2 schedule marks the accessory dwelling row P (permitted) in U, AG, RA, RS, RD, RML, RMH and RP. Only one ADU is allowed per lot (10-29-3.D), it must be accessory and subordinate to a single-family home on the same lot (10-29-3.B), and a detached unit follows the zone's accessory-structure bulk rules for setbacks, easements and height (10-29-3.G). Mobile homes, RVs and tiny homes on wheels cannot serve as an ADU (10-29-3.H).

Since Ordinance No. 4948 (adopted September 21, 2026) Nampa allows an attached or internal ADU, not only a detached one: 10-29-3.C reads 'An accessory dwelling unit may be internal to, attached to, or detached from a stand alone structure', and the old detached-only definition at 10-1-2.H (500 square feet, one bedroom) was repealed and replaced with the state's definition of a self-contained living unit with its own cooking, sleeping and sanitation facilities on the same lot as a single-family home. The same size limits apply as for a detached unit, and the chapter adds a guardrail: 'Any Accessory Dwelling Unit proposed to be greater than 1,000 s.f. and attached to the primary dwelling unit will be defined as a duplex', which needs a conditional use permit in the RS zone and is not allowed in AG or RA. The building code side (Title 4, which now adopts the 2024 IRC Appendix BC for ADUs) adds a separate entrance, a separate egress path and fire or alarm separation between the two units.

Section 10-29-3.I, adopted by Ordinance No. 4948 on September 21, 2026, sets the cap by the size of the main house: 'For properties with a primary dwelling unit square footage of 1,333 s.f. or less, an ADU that measures 75% of the primary dwelling unit's overall, existing square footage, may be constructed. For properties with a primary dwelling square footage of 1,334 s.f. or greater, the owner may construct an ADU that measures up to 1,000 sq.ft. total. (square footage = overall structure, not just footprint).' That replaces the old 500 square foot, one-bedroom limit. A detached unit can still be squeezed by the general accessory-structure rules: 10-1-7.D.2 caps the combined footprint of a property's detached accessory structures at the footprint of the house, and detached accessory structures may not be taller than the house or 22 feet, whichever is greater (10-8-4.A in RS). The adopted building code appendix also sets a 190 square foot minimum.

Nampa's ADU chapter says so outright: 10-29-3.F reads 'Owner occupancy of the lot or parcel is not required.' Neither the owner nor anyone else has to live in the main house or the ADU, at application or afterward. Idaho Code 67-6541(2)(e) separately requires cities over 10,000 people to prohibit owner-occupancy requirements for either unit, so one cannot be added back later.

Section 10-29-4, adopted by Ordinance No. 4948 on September 21, 2026, reads 'The ADU shall provide 1 off-street parking space, in addition to the required parking for the primary dwelling, unless one of the following exceptions apply': the property already provides the code-required off-street spaces for the main house (10-29-4.A), the adjacent street is designed and constructed to allow on-street parking (10-29-4.B), or the main house is within one-quarter mile of transit, an employment center or commercial services (10-29-4.C). Because any one exception is enough, most ordinary single-family lots with a standard two-car driveway or garage owe no extra space. This ADU-specific rule replaces the old ambiguity in which the RS zone's two-spaces-per-dwelling-unit text (10-8-9) could be read onto an ADU. It is no stricter than Idaho Code 67-6541(2)(b), which lets a city require ADU parking only where the house has no off-street parking, the street is not built for on-street parking, or the house is within a quarter mile of transit, an employment area or commercial services.

An ADU is a permitted use with no public hearing. Section 10-29-3.A states 'An ADU is permitted within residential zones as defined in 10-3-2', the 10-3-2 schedule marks the accessory dwelling row P, and its key reserves the chapter 25 conditional use permit for uses marked C. Section 10-29-5 routes every ADU through 'an application for a Nampa Building Permit' reviewed for building and fire codes, flood plain rules, utility connections, drainage, access, rights-of-way and easements, and Nampa publishes a dedicated Residential Accessory Dwelling Unit Building Permit Application. Idaho Code 67-6541(3) also requires conforming ADU projects to be 'approved administratively and as a matter of right, without the need for discretionary approval'. Standard plan review, permit, impact and utility hook-up fees apply (10-29-6).

Nampa has no rental license, no cap on renting an ADU and no residency test, and the new ADU chapter makes the unit easier to lease as a separate home: 10-29-3.E has the city's GIS division assign the ADU its own address, and the old one-bedroom, no-separate-address, no-separate-utilities limits were repealed with the 500 square foot definition. Since July 1, 2026 Idaho Code 67-6539 bars any city from prohibiting short-term rentals or imposing owner occupation, day limits, extra parking, caps, spacing rules or a conditional use permit in residential zones, and subsection (5) states that no city 'shall require a license, fee, permit, certification, or registration to operate a short-term rental'. The ADU cannot be split off and sold, since 10-29-3.B keeps it accessory to a single-family home on the same lot, and 10-3-2 note 1.c bars one where multiple dwelling units already exist and requires it to be removed or integrated if two or more units are later built.